PostgreSQL用户通过md5加密时，加了salt，而这个salt是用户名字符串。
二、源码分析
CreateRole：
shadow_pass = encrypt_password(Password_encryption, stmt->role,password);
|-- pg_md5_encrypt(password, role, strlen(role),encrypted_password);
| |-- memcpy(crypt_buf, passwd, passwd_len);
| | memcpy(crypt_buf + passwd_len, role, strlen(role));
| | strcpy(buf, "md5");
|-- |-- pg_md5_hash(crypt_buf, passwd_len + salt_len, buf + 3);
new_record[Anum_pg_authid_rolpassword - 1] =CStringGetTextDatum(shadow_pass);
三、gdb跟踪
1、在函数encrypt_password上打断点，然后客户端执行：create user yzs with password '123456';创建带密码的用户，观察是否默认使用md5。
postgres=# create user yzs with password '123456';
2、堆栈信息
Breakpoint 1, encrypt_password (target_type=PASSWORD_TYPE_MD5, role=0x99c3b3c "yzs", password=0x99c3b4c "123456") at crypt.c:111
111 PasswordType guessed_type = get_password_type(password);
(gdb) bt
#0 encrypt_password (target_type=PASSWORD_TYPE_MD5, role=0x99c3b3c "yzs", password=0x99c3b4c "123456") at crypt.c:111
#1 0x0827b7a2 in CreateRole (pstate=0x9a0d804, stmt=0x99c3bbc) at user.c:412
#2 0x0840fc18 in standard_ProcessUtility (pstmt=0x99c3c14, queryString=0x99c31dc "create user yzs with password '123456';", context=PROCESS_UTILITY_TOPLEVEL, params=0x0, queryEnv=0x0,
dest=0x99c3d74, completionTag=0xbf9119e6 "") at utility.c:722
#3 0x0840f42a in ProcessUtility (pstmt=0x99c3c14, queryString=0x99c31dc "create user yzs with password '123456';", context=PROCESS_UTILITY_TOPLEVEL, params=0x0, queryEnv=0x0,
dest=0x99c3d74, completionTag=0xbf9119e6 "") at utility.c:357
#4 0x0840e6ea in PortalRunUtility (portal=0x9a20634, pstmt=0x99c3c14, isTopLevel=1 '\001', setHoldSnapshot=0 '\000', dest=0x99c3d74, completionTag=0xbf9119e6 "") at pquery.c:1178
#5 0x0840e8b7 in PortalRunMulti (portal=0x9a20634, isTopLevel=1 '\001', setHoldSnapshot=0 '\000', dest=0x99c3d74, altdest=0x99c3d74, completionTag=0xbf9119e6 "") at pquery.c:1324
#6 0x0840ded2 in PortalRun (portal=0x9a20634, count=2147483647, isTopLevel=1 '\001', run_once=1 '\001', dest=0x99c3d74, altdest=0x99c3d74, completionTag=0xbf9119e6 "") at pquery.c:799
#7 0x08408692 in exec_simple_query (query_string=0x99c31dc "create user yzs with password '123456';") at postgres.c:1099
#8 0x0840c5d4 in PostgresMain (argc=1, argv=0x997edc4, dbname=0x997ecf4 "postgres", username=0x99558cc "postgres") at postgres.c:4088
#9 0x083864e6 in BackendRun (port=0x9978038) at postmaster.c:4409
#10 0x08385c5d in BackendStartup (port=0x9978038) at postmaster.c:4081
#11 0x083822d9 in ServerLoop () at postmaster.c:1755
#12 0x083819d6 in PostmasterMain (argc=3, argv=0x9953810) at postmaster.c:1363
#13 0x082dfb60 in main (argc=3, argv=0x9953810) at main.c:228
(gdb) n
114 if (guessed_type != PASSWORD_TYPE_PLAINTEXT)
(gdb)
123 switch (target_type)
(gdb)
126 encrypted_password = palloc(MD5_PASSWD_LEN + 1);
(gdb)
128 if (!pg_md5_encrypt(password, role, strlen(role),
(gdb) s
pg_md5_encrypt (passwd=0x99c3b4c "123456", salt=0x99c3b3c "yzs", salt_len=3, buf=0x9a0d984 "") at md5.c:326
326 size_t passwd_len = strlen(passwd);
(gdb) n
329 char *crypt_buf = malloc(passwd_len + salt_len + 1);
(gdb)
332 if (!crypt_buf)
(gdb)
339 memcpy(crypt_buf, passwd, passwd_len);
(gdb)
340 memcpy(crypt_buf + passwd_len, salt, salt_len);
(gdb) p crypt_buf
$1 = 0x9979e68 "123456A"
(gdb) n
342 strcpy(buf, "md5");
(gdb) p crypt_buf
$2 = 0x9979e68 "123456yzsנ\tQ"
(gdb) n
343 ret = pg_md5_hash(crypt_buf, passwd_len + salt_len, buf + 3);
(gdb) p crypt_buf
$3 = 0x9979e68 "123456yzsנ\tQ"
(gdb) n
345 free(crypt_buf);
(gdb) p crypt_buf
$4 = 0x9979e68 "123456yzsנ\tQ"
(gdb) n
347 return ret;
(gdb)
348 }
(gdb) p ret
$5 = 1 '\001'
(gdb) n
encrypt_password (target_type=PASSWORD_TYPE_MD5, role=0x99c3b3c "yzs", password=0x99c3b4c "123456") at crypt.c:131
131 return encrypted_password;
(gdb)
146 }
(gdb)
CreateRole (pstate=0x9a0d804, stmt=0x99c3bbc) at user.c:415
415 CStringGetTextDatum(shadow_pass);
(gdb) p shadow_pass
$6 = 0x9a0d984 "md5aed8080c314507e15542d5e9519723a8"
3、从pg_authid表中观察该用户经过md5加过密的密码值，可以看出和堆栈信息中看到的一样